ENERGY FROM OUR NEAREST STAR

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BUILDING AN EXPERIMENTAL SOLAR HEATING SYSTEM

LOTHAR GRAUDINS, Ph.D JOHN FLEMING Copyright 2009

You have probably noticed that a garden hose left in the sun (after you've shut the water off) eventually contains some very hot water. Or, a chrome-plated wrench left in the sun actually becomes too hot to handle. Simply put, our sun continuously produces a tremendous amount of energy, not only in the visible spectrum but shorter wavelengths, ultraviolet, as well as longer wavelengths known as infrared. The challenge is how to make use of this energy that can potentially be used for heating or cooling. I have called our system experimental because, at the time of building it, there was limited technical information available. Also, this system was built to be flexible. It can be expanded to incorporate other uses, such as directly heating a hot water heater or hot tub. Moreover, due to the high cost of commercial water panels, we intend to build our own panels and incorporate these into the system.(More about that later). Finally, environmental conditions such a fog or excessive cloudiness will impact the overall performance. In this regard, it is always necessary to have an alternate heating system. I use a small conventional propane furnace and a wood-burning stove. A good back-up is a hot water heater running on gas. As the panels fail to support the floor temperature, the water heater takes over. One of many possibilities for a solar heating system is to build a hydronic system consisting of Rehau Brand Pex tubing. A hydronic system means a pattern or circuit of tubing, usually installed under the floor. “Pex” is very tough, yet flexible (cross-linked polyethylene) plastic tubing. The tubing is also freeze resistant and tolerates hot water. It is a superior product in most plumbing applications and comes with a complement of fittings and joints. When we first built this system, initially for a 1,200 ft. workshop, we installed the Pex tubing directly under (inside) the concrete slab. Although concrete has only about 20 percent of the heat capacity of water, i.e., water can hold 5 times the heat energy, a large slab makes for a sizable heat storage system. Nevertheless, all things considered, installing the tubing above an insulated slab is preferred. (The installation is covered with a wood floor.) In either case, there is a decided advantage over conventional heating. Heat rises upward from the floor and no circulation system is needed. Fans that are used by conventional systems are often noisy and blow dust and pollen around. Incidentally, Rehau also has some solid engineering data available as to practical application of Pex tubing for a hydronic system. Take a look at the simplified illustration on page 7 (Figure 1). Sunlight enters the panels from a southern direction. Once the panel temperature exceeds the floor temperature (a controller, a Goldline GL-30 was used) the circulation pump starts. It pumps the hot water from a storage tank into the floor loops. A return pipe leads into the bottom of the panels, feeding the cooled water back into the closed system. Page 1

Before beginning a solar project, we want to know approximately, if not as accurately as possible, how much energy we can expect from the sun. We are interested in knowing the efficiency with which we can harness this energy. Finally, we also want to know how much energy our proposed project might require. Let's take look at some of the parameters of interest that will facilitate making such measurements. We start with a “constant” known as the “Solar Constant.” In reality, a complex variable, but it does give us a starting point for solar applications. It has been calculated that the sun's total output of radiant energy produces about 1,366 watts per square meter. Of course, this represents a maximum, measured by satellites and before impacting the atmosphere. The watt is a unit of energy, technically defined as the energy expended by one
amp driven across a potential of one volt. In a more familiar context, a common 60 watt light bulb operating in a 120 volt circuit that is supplying .5 amps of power expends 60 watts. To calculate watts, you multiply amps times volts.

If we start with 1,366 watts per square meter, we can calculate the number of watts per square foot: 1,366 watts/ sq. meter / 10.76 sq. feet/ sq. meter) = 127 watts per square foot. As previously stated, this represents available radiant power prior to entering the atmosphere. Studies show, on average, that the sun's radiant energy is attenuated by 23 per cent in passing through the atmosphere. Or, conversely, we retain about 77 per cent of what is available. In keeping with our dimensions, 77 per cent of 127 watts per square foot is 98 watts per square foot. Or, if you prefer, this amounts to about 98 watts per sq. ft. X 10.76 sq. ft./sq. meter = 1,054, roughly a 1,000 watts/ sq. meter. The second parameter of convenient use is the British Thermal Unit or BTU. It requires 1 BTU (or unit of power) to raise the temperature of 1 pound of water to 1 degree F. Given, for example, that we wanted to raise the temperature of 5 gallons of water from 40 degrees F. to 90 degrees F., we calculate: 5 gallons (8.3 lbs./gallon) = 41.5 lbs. 90 degrees – 40 degrees= 50 degrees BTU's required: (41.5)(50) = 2,075 page 2

In terms of watt-hours, 2,075 BTU's is equivalent to about 608 Watts. This means that 1 watt-hour is equal to about 3.41 BTU's. A watt-hour is the energy expended by 1 watt during the course of one hour. We typically rate appliances in terms of kW or Kilo (1,000) watt consumption. The system we built was designed to be a “drain-back” system. Such a gravity dependent system is more reliable than, for example, solenoid valves. A drain-back system is insurance against freezing occurring within the panels. Once the circulating pump shuts off, the panels drain into a small holding tank. Also, since the total volume of water is small, we added antifreeze. The antifreeze used is propylene glycol, the kind used in RV systems. A larger system would also benefit from the drain-back feature. In place of antifreeze however, we recommend “heat-wrap” on vulnerable copper tubing. We started with 4 used Revere brand commercial panels, measuring 77 X 35 inches each. Each panel, measuring 18.7 square feet (a total of about 75 square feet). Using the conversion factor of 3.41 BTU's per watt: 3.41 BTU's X 98 Watts/ sq. foot X 75 square feet=25,064 BTU's This figure, 25,064 BTU's is the theoretical amount of energy received by the 4 panels in question. However, there are considerable losses. For one, despite the use of selective absorbents, a significant amount of energy is simply reflected from the panels into the surrounding atmosphere. Energy in the form of heat is also released. Panels of this nature seem to operate between 40 and 60 percent efficiency, depending upon panel temperature. If we assume an efficiency of 60 % (rating for these particular commercial panels) we end up with about : (.60) (25, 064 BTU's) = 15, 038 BTU's

Over a period of 6 hours of bright sunlight, we expect a total daily total of 90,228 BTU's of energy. This energy, in the form of hot water is transported into the building and released into the concrete floor. Considering the BTU's of one gallon of propane rated at 91,330, about 1 gallon is saved daily. Since the building measures 30 X 40 or 1,200 square feet and estimating 4 inches of concrete, we calculate a total of 400 cubic feet of concrete. The particular aggregate of concrete used weighs about 150 lbs. per cubic foot. Thus, the total weight of the floor is 60,000 lbs. Since the heat capacity of concrete is about 1/5 that of water or .21, it takes .21BTU's to raise the temperature of 1 lb. of concrete 1 degree Fahrenheit. We can calculate the theoretical amount of energy needed to raise the temperature of 60,000 lbs. of concrete by 1 degree F(.21 BTU's ) (60,000 lbs. Concrete) = 12, 600 BTU's page 3

This figure, 12,600 BTU's is the theoretical amount of energy needed to raise the temperature of the floor by 1 degree F. Of course, there will be losses. For example, in our case, the floor is not insulated on the edges. At first, by looking at the above figures, it would appear that if I could expect about a 68 degree change in the floor temperature in the course of a sunny day. This is a good estimate for typical fall weather. I don't use the system in the summer, a time of optimal conditions. Nevada has sufficiently high temperatures during the summer months, often in the 90's. During the fall, daytime temperatures are often in the low 70's but followed by cold nights (30-40 degrees F). Since cold mornings contribute to a lower overall gain, performance is reduced. Still, a 5-6 degree change is typical, with occasional 8-10 degree changes on a particularly warm day, (72F+) outside temperature. In relatively warm Fall conditions, a loss of 6-7 degrees F. of room temperature overnite is typical. This is a well-insulated building, R-30 on walls and ceilings. The floor is insulated (underneath) with R-7. Obviously, since the outside temperature might fall to 35 F, the floor is acting as a heat capacitor or storage system. During winter months, the solar heat system keeps the building from freezing: rarely does the inside temperature fall below 40 degrees. That is certainly one advantage of this installed system! During the summer months, this system could accommodate a hot tub as well. The electrical bill often shows an extra $50.00 per month for electrically heating a hot tub. It should be pointed out that this system is presently limited by too few panels and the lack of a sizable storage tank. Since water has the highest heat capacity of common materials, a large and well- insulated storage tank would add to the reserve of thermal energy. One suggestion is to have about two gallons of water per square foot of collector area: 2 gallons water (75 square feet of panel) = 150 gallons I would recommend a larger storage tank. Consider that a 2 X 3 X 8 foot tank (2 feet deep) would hold 48 cubic feet or nearly 360 gallons. This more than doubles the suggested capacity. We would like to have at least enough storage to take care of 2 consecutive cloudy days. Commercial panels are quite expensive but can be built. The heart of the panel is a sheet of copper connected to copper tubing running vertically over its surface. ( See Figure 2, p.8). Above and below the tubing arrangement are copper manifolds (1 inch copper tubing). Once you have soldered the tubing arrangement to the copper panel, clean the surfaces with some dilute Sulfuric Acid. Rinse and allow to dry. Next prepare a hot concentrated solution of Potassium Chlorate. Page 4

Brush this solution over the entire front of the panel to turn the copper surface into an oxidized (black) patina. This serves as a selective absorber. Or, in other words, a material that does not readily re-emit infrared energy back out of the panel. This works amazingly well, so much so that uncirculated water will come to a boil inside the panel. The panel is well insulated. It is housed in a wood frame, fitted with an iron-free window. For this, look for discarded sliding glass patio doors. Glass shops often have these on hand for very little cost. Since this tempered glass cannot be cut, design your panel to match the available size. If you wish, an iron panel can also be used. You can research this on the net. Incidentally, you can also choose to buy the bare panel and finish it yourself. Instead of the unwieldy glass cover, a plastic sheet of insulated (twin-wall) polycarbonate may be used. This polycarbonate sheet is typically used in greenhouse construction. The circulating pump installed in this system and recommended is a stainless steel Taco 007 circulation pump. The distribution system, if you have multiple loops, is a manifold with individual faucets that control the flow to each loop. As previously stated, you can use “heat tape” and avoid antifreeze. This would be ideal for adding a hot tub heating loop. By circulating hot water directly from the panel, you can avoid a less efficient higher temperature heat-exchange system. Important points: 1. Insulate all tubing. Good insulation is the key to efficiency. The same principle is important for your home insulation, during winter or summer. 2. Use a drain-back system. 3. Remember to use sloping on the plumbing system. To purge air, use 1/8 inch/foot sloping on panel headers. 4. The panel rack must be rigorously constructed to withstand high winds. Use tiedowns. 5. After doing basic calculations, overbuild your system. Be flexible and always have a back-up system, preferably connected to, but operating independently of your solar system. 6. Solar Heating is not cheap, but on the brighter side, the costs can usually be recaptured in terms of energy savings. It also depends on how much of the system you will build or install yourself. Also, many states currently allow for tax credits.

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References: An excellent article on Solar Heating can be found in Mother Earth News, Issue No. 225, December 2007/January 2008, p. 36. This article presents important details on the construction of a sizable system, together with some practical measurements of performance. The Control Diagram found online is great for an overview of the whole system.